Method for signaling quality of range estimates in UWB devices

ABSTRACT

A method signals a quality of range estimates in a UWB network. For each range estimate, a confidence level of a range estimate is signaled, a confidence interval for the range estimate is signaled, and a confidence interval scaling factor for the confidence interval is signaled.

FIELD OF THE INVENTION

This invention relates generally to radio ranging, and more particularlyto ranging with ultra wideband radio signals.

BACKGROUND OF THE INVENTION

There is a growing demand for location awareness in short range radionetworks, particularly in ultra wideband (UWB) networks. Typically, thelocation of a node in the network is determined based radio rangingmeasurements.

UWB or digital pulse wireless communication is a wireless technology fortransmitting large amounts of data over a wide spectrum of frequencybands with very low power and for a short distance. UWB radio signalsnot only can carry a huge amount of data over a short distance at verylow power, e.g., less than 0.5 milliwatts, but have the ability to carrysignals through doors and other obstacles that tend to reflect signalshaving more limited bandwidths and a higher power.

UWB signals are transmitted as digital pulses that are timed veryprecisely on a carrier signal across a very wide spectrum offrequencies. A transmitter and a receiver are synchronized to send andreceive pulses with an accuracy of trillionths of a second. On anyparticular frequency, the UWB signal has less power than normal andanticipated background noise. Theoretically, interference withconventional radio signals is negligible.

UWB communication has three main types of application. In radarapplications, the UWB signal penetrates nearby surfaces but is reflectedby surfaces that are farther away, allowing objects to be detectedbehind walls or other coverings. In data transmission applications,digital pulses allow a very low powered and relatively low cost signalto carry information at very high data rates over a short range. Inlocation awareness applications, ultra wideband digital pulses allowaccurate ranging estimate between different devices.

UWB applications communicate in accordance with a protocol stack thatincludes a physical layer (PHY), a media access control (MAC) layer, anetwork layer, a transport layer, a session layer, a presentation layer,and an application layer.

UWB two-way ranging is performed by two transceivers. Conventionally, arange packet is sent from a device A to a device B. Upon receipt of therange packet at the device B, the range packet is returned to device A.Measuring the length of time required for this roundtrip can reveal thedistance between the two transceivers.

For example, a transmitter can send a signal to a receiver at t₁. Thereceiver, as soon as possible, returns a reply signal to thetransmitter. The transmitter measures the time of arrival (TOA) of thereply signal at time t₂. An estimate of the distance between thetransmitter and the receiver is the time for the signal to make theround trip divided by two and multiplying by the speed of light is,i.e.:

$D = {\frac{{t_{1} - t_{2}}}{2}{c.}}$

To meet the need for improved and private location awareness in UWB, anIEEE 802.15.4a Task Group (TG) has been established to develop aUWB-based physical (PHY) layer standard with a precision rangingcapability. An UWB signal has a relative bandwidth larger than 20%, oran absolute bandwidth of at least 500 MHz. One type of an UWB system isan impulse radio (IR). IR uses extremely short duration pulses togenerate signal waveforms, and allows fine time resolution of channelmultipath characteristics, which is important in identifying the line ofsight signal for precision ranging.

Various publications have described ways to accurately estimate thedistance between two devices. In a paper by J-Y. Lee and R. A. Scholtz,“Ranging in a dense multipath environment using an UWB radio link,” IEEETrans. Select Areas in Communications, vol. 20, issue 9, pp. 1677-1683,Dec. 2002, the entire contents of which is incorporated by reference, atime-of-arrival (TOA)-based ranging scheme using an ultra-wideband (UWB)radio link is described. That ranging scheme implements a search processfor the detection of a direct path signal in the presence of densemultipath, utilizing generalized maximum-likelihood (GML) estimation.Models for critical parameters in the process are based on statisticalanalysis of propagation data. The process is tested on anotherindependent set of propagation measurements. That UWB ranging systemuses a correlator and a parallel sampler with a high-speed measurementcapability in the transceiver to accomplish two-way ranging in theabsence of synchronized clocks. In a paper by S. Gezici, Z. Tian, G. B.Giannakis, H. Kobayashi, A. M. Molisch, H. V Poor, Z. Sahinoglu,“Localization Via UWB Radios,” IEEE Signal Pro. Magazine, v. 22, n. 4,pp. 70-84, Jul. 2005, the entire contents of which is incorporated byreference, localization techniques relying on wireless ultra-wideband(UWB) signaling are described. Various localization alternatives areconsidered and the UWB time-of-arrival based one is found to have ahighest ranging accuracy.

A further important step is to derive the position (location) of a node(device) A from the estimates of the ranges between this device A andother nodes. Using three or more such range estimates, the position(relative to the other nodes) can be determined. If the ranges are knownideally, then the position estimate also is perfect, and it does notmatter whether three or more range estimates are present. Additionalrange estimates, e.g., more than three, just confirm the positionestimate. However, in practice, the accuracy of the range estimate isalways limited. In that case, a larger number of range estimates helpsto decrease the error in the position estimate. Different combinationsof range estimates result in different position estimates, and combiningthose different position estimates improves the overall accuracy. Whenusing that technique, it is important to know the reliability of thedifferent range estimates, and this reliability has to be communicatedthrough the network to the nodes that make the actual positionestimates.

Communicating the reliability of range estimates in an efficient way isthus important, but nontrivial. Ideally, the probability densityfunction (pdf) of the range estimate should be communicated. However, inorder to reduce the overhead, limited information can be transmittedover the network and the transmission occurs digitally. Therefore,quantization has to occur.

A conventional way for quantizing pdfs is to express the pdfs inparametric form, and communicate the suitably quantized parametersthrough the network. A simple example of that is a description of aGaussian pdf, where only the mean and the variance has to be signaled.However, no parametric form of the range estimate pdf is known; it isonly established that the pdf is not Gaussian. Therefore the parametricrepresentation cannot easily be applied to ranging data.

Therefore, the current state of the art defines nominal intervalsdescribing the accuracy of the estimate (henceforth called confidenceintervals) and signals the level of confidence into each of them. Forexample, a proposal from TimeDomain Corporation for the IEEE 802.15.4astandard defines a 5-bit range quality indication, see Vern Brethour,“Ranging Values,” IEEE 802.15-05-0679-01-004a, incorporated herein byreference. Two bits are used for a confidence interval, and three bitsare used for a confidence level. The possible range resolution is verysmall, because only two bits are used to indicate the confidenceinterval.

The requirements for range accuracy can vary widely, depending on theapplications. For line-of-sight situations with high transmissionbandwidth, e.g., 7.5 GHz, range accuracies of less than 1 cm aredesired. For other situations, e.g., non-LOS, non-coherent receivers,and distances between nodes larger than 10 meters, range accuracies ofmore than 1 meter are desired. Therefore, the traditional methodrequires the definition and signaling of a large number of ranges, whichin turns requires the transmission of a large number of bits.

SUMMARY OF THE INVENTION

The embodiments of the invention provides a scaling factor so thatsignaled values of confidence intervals multiplied by this factor givethe intended values of the confidence intervals. For example, if thenominal signaled value of the confidence interval is 1 ns and thescaling factor is 0.05, then the confidence interval becomes 0.05 ns.One scaling factor can be valid for multiple confidence intervals. Thevalue of scaling factor can be sent for a specific link or distributedthroughout the network

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of quality signals for radio range estimatesaccording to an embodiment of the invention;

FIG. 2 is a flow diagram of a method of signaling quality of rangeestimates according to an embodiment of the invention; and

FIG. 3 is a block diagram of a network for radio ranging according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a wireless network for signaling ranges according to anembodiment of the invention. The network includes devices 310 and 320.The devices can be implemented as radio transceivers. In a preferredembodiment of the invention, the devices exchange ultra wideband signals(UWB). Device A 310 sends a range packet 330. In response to receivingthe range packet, device B 320 responds with another range packet 340.Similarly, device B 320 can send a range packet 340 and device A 310receives and sends another ranging packet 330. Measuring the time tosend and receive the range packets reveals a distance 350 between thedevices. The devices can also exchange a range quality indicator 100indicative of the accuracy of the range estimate.

The number of required different ranges for a specific application istypically very small. If a minimum accuracy of interest is, for example,0.1 cm, then any estimate with an error of larger than 10 cm isconsidered erroneous and the range estimate is discarded. On the otherhand, if the minimum range of interest is 10 cm, then it is required todistinguish between a 10 cm error and a 100 cm error.

Most standards, including the IEEE 802.15.4a standard, are intended fora number of different applications. Covering all possible estimationranges of interest would require a large number of confidence intervals.

Therefore, we first define confidence intervals, for a typical case,e.g., with a minimum accuracy of interest equal to 10 cm. Furthermore,we provide a scaling factor that is signaled with extra bits, e.g., twobits. Other sized scaling factors can also be used. This scaling factorscales the nominal value of all defined confidence intervals by thescaling factor, e.g., a scaling factor of 10. Therefore, depending onthe scaling factor, which in turn can be selected depending on theapplication, the confidence intervals have different meanings.

Therefore, we modify the “standard approach” as described in the priorart, by adding the scaling factor.

The scaling factor can be selected according to the range qualityestimate, or simply based on the application for which the wirelessnetwork is designed. In the former case, the scaling factor is selectedadaptively, i.e., can change in the case of a tie, while in the lattercase, the scaling factor might be set once e.g., at the factory, or bythe user, or a higher signaling layer, and then remain unchanged.

As shown in FIG. 1, a range quality indicator 100 for a range estimateincludes the following.

A figure of merit (FoM) confidence level 101 is signaled with threebits. As an example, those three bits can represent the followingconfidence intervals 0%, 20%, 55%, 75%, 85%, 92%, 97%, and 99%. Ofcourse, other values are possible as well.

A FoM confidence interval 102 is signaled with two bits. As an example,the values are 3 cm, 9 cm, 30 cm, and 90 cm.

A FoM confidence interval scaling factor 103 is signaled with two bits.As an example, the scaling factor can represent 0.5, 1, 2, and 4.

FIG. 2 shows the general method for signaling the quality of rangeestimates 240, including the quality indicator 100. The range quality isestimated 210 and influences the selection of the scaling factor 220.After selecting the confidence interval 230, the range quality indicator100 is formatted for signaling range quality estimates 240.

Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications can be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

1. A method for signaling a range quality estimate a in a wireless network, comprising for each range estimate: transmitting a first range packet from a first device to a second device; transmitting a second range packet from the second device to the first device in response to receiving the first range packet by the second device; determining the range quality estimate based on the first range packet and the second range packet; wherein the range quality estimate is indicative of an accuracy of a range estimate; exchanging the range quality estimate between the first device and the second device, wherein the range quality estimate includes a confidence level of the range estimate, a confidence interval for the range estimate, and a confidence interval scaling factor for the confidence interval.
 2. The method of claim 1, further comprising: providing the scaling factor value over the network.
 3. The method of claim 1, further comprising: signaling the confidence level with three bits; signaling the confidence interval with two bits; and signaling the scaling factor with two bits.
 4. The method of claim 1, in which the scaling factor scales the range confidence intervals.
 5. The method of claim 1, in which the scaling factor of confidence interval depends on a particular application.
 6. The method of claim 1, in which the scaling factor of the confidence interval depends on the quality of range estimates.
 7. The method of claim 1, in which the network uses ultra wideband signals.
 8. The method of claim 1, in which the transmitting and receiving, and exchanging is according to an IEEE 802.15.4a standard.
 9. A system for signaling a range quality estimate in an ultra wideband network, comprising for each range estimate: a first device configured to transmit a first range packet; a second device configured to transmit a second range packet in response to receiving the first range packet; means for determining the range quality estimate based on the first range packet and the second range packet; wherein the range quality estimate is indicative of an accuracy of a range estimate; means for exchanging the range quality estimate between the first device and the second device, wherein the range quality estimate includes a confidence level of the range estimate, a confidence interval for the range estimate, and a confidence interval scaling factor for the confidence interval. 